Keep PostgreSQL from sometimes choosing a bad query plan

Question

I have a strange problem with PostgreSQL performance for a query, using PostgreSQL 8.4.9. This query is selecting a set of points within a 3D volume, using a LEFT OUT

Answer 1

What Erwin said about the statistics. Also:

ORDER BY parentid DESC, id, z_diff

Sorting on

parentid DESC, id, z

might give the optimiser a bit more room to shuffle. (I don't think it will matter much since it is the last term, and the sort is not that expensive, but you could give it a try)